α-HC(NH2)2PbI3 (α-FAPbI3) perovskite films are highly desirable for perovskite solar cells (PSCs) due to their enhanced sunlight absorption extending into the infrared (IR). However, the thin-film deposition of (α-FAPbI3) perovskite is significantly more challenging compared to its CH3NH3PbI3 (MAPbI3) perovskite counterpart.
Films of organolead trihalide perovskites have been studied extensively as light-absorbing materials, which are at the heart of the new perovskite solar cells (PSCs). The unique combination of low-cost solution-processing, and high power-conversion efficiencies (PCEs) rivaling those of conventional Si-based solar cells, holds great promise for PSCs. While methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) perovskite, with a bandgap of ˜1.55 eV, is the most widely studied in the context of PSCs, formamidinium lead triiodide (α-HC(NH2)2PbI3 or α-FAPbI3) perovskite for PSCs is also very promising. This is primarily because FAPbI3 has a smaller band gap of ˜1.45 eV, extending light absorption into the infrared region of the solar spectrum.
Since the morphology of perovskite films plays a central role in determining the PCE of PSCs, unprecedented efforts have been made in order to control the film uniformity and/or tailor the perovskite microstructures, with particular emphasis on MAPbI3 perovskite. However, the development of effective protocols for the engineering of FAPbI3 perovskite film morphologies and microstructures are lagging due to the following challenges. First, the ‘ionic radius’ of FA+ cation (2.79 Å) is larger than that of MA+ cation (2.70 Å), and the molecular structures of FA+ and MA+ cations are quite different, both of which are expected to affect solution-growth kinetics of α-FAPbI3 perovskite. Second, FAPbI3 also crystallizes in a ‘yellow’ δ-FAPbI3 non-perovskite polymorph at room temperature, which is associated with the formation of the α-FAPbI3 perovskite. Therefore, the growth of phase-pure α-FAPbI3 perovskite films requires stricter control over the synthetic procedures compared with MAPbI3, which is a major hurdle in the path of realizing the full potential of α-FAPbI3 perovskite for PSCs. Thus, there is a need for methods that overcome these difficulties to produce high quality α-FAPbI3 perovskite films suitable for PSCs.